The present invention relates to wavelength division multiplexers (WDMs), particularly WDMs utilizing interferometric devices capable of channel separation, combination, and routing.
Wavelength division multiplexing is widely used in fiber optic communication systems to increase the transmission capacity of networks by allowing multiple carrier wavelengths, or channels, to be transmitted and received over a single optical fiber. In addition, WDMs can be used in fiber optic communication systems for other purposes, such as dispersion compensation, noise reduction, EFDA and Raman amplification, and gain flattening.
Optical wavelength division multiplexers receive two or more individual wavelengths (also referred to as colors or frequency channels) and combine them into one signal in a waveguide. Optical wavelength division demultiplexers receive an optical signal with two or more wavelengths from a waveguide and separate the optical signal into its component frequencies.
A wide variety of technologies are used to manufacture WDMs. Examples include dielectric thin film filters, fiber Bragg gratings, fused biconical tapers, and Arrayed Waveguide Gratings.
FIG. 1 is a diagram illustrating a prior art WDM using a conventional dielectric thin film filter. A multilayer interference film 100 is deposited on transparent substrate 110 to form a narrow bandpass filter. Collimated light 120 carrying a plurality of wavelengths λ1-λn is incident on film 100. Optical conditions determined by the characteristics of film 100 allow only light 140 having wavelength λk to pass through, while the other wavelengths are reflected. Cascading these thin film filters can make a WDM capable of separating more than one wavelength. With increasing channel counts, filters are cascaded serially to separate the channels. Insertion losses accumulate in the WDM, which attenuates channels at the end of the filter chain and causes nonuniform losses among the channels. In addition, with current thin film technology, the channel spacing for practical purposes is limited to greater than 200 GHz (1.6 mn at 1550 nm), which limits the channel capacity of fiber optical network systems.
FIG. 2 is a diagram representing a conventional fiber Bragg grating. A fiber Bragg grating is fabricated by inscribing Bragg gratings directly into a photosensitive fiber core using UV light and photomasking. With proper design, a fiber Bragg grating 200 reflects the selected wavelength while transmitting other wavelengths. A circulator 210 is typically used to extract the reflected wavelength. Circulators tend to be very expensive. Alternately, a fiber Mach-Zehnder interferometer (MZI) can be used. In the past MZIs have been difficult to align and keep aligned in the presence of temperature variations and vibrations. Furthermore, since fiber Bragg gratings only filter out one wavelength at a time, they must be cascaded in a serial fashion to separate channels. As with thin film filters, insertion losses accumulate in the WDM, which attenuates channels at the end of the filter chain and causes nonuniform losses among the channels.
FIG. 3 is a diagram showing a conventional single-mode fused biconical taper (FBT) fiber coupler that can also be used to form WDMs. Typically, two single-mode optical fibers are fused together and elongated to reduce the core size, which enlarges the mode size and moves the two fiber cores closer to form a fused fiber coupler 300. Signals of two different wavelengths, for example 1310 nm and 1550 nm, enter the input terminal 310 and are separated into the first and second fibers 330 and 340 of the output terminal. The coupling ratios for the two channels exhibit complementary sinusoidal behavior for amplitude as a function of frequency within the passband of the WDM, with each channel having one or more peaks or windows within the passband. Unfortunately, the FBT coupler is only suited for separating channels whose wavelengths are relatively far apart. To achieve the multi-window WDMs or small channel spacing, it is necessary to significantly increase the length of the fused and tapered region, which has presented significant difficulties in manufacturing.
FIG. 4 is a diagram illustrating a conventional arrayed waveguide grating (AWG) 400 capable of splitting wavelength-multiplexed light. Light 410 including a plurality of different wavelengths enters the receiving end of the AWG 420 and is divided into a number of waveguides 430 each having different optical path lengths. At the end of the grating array 440, optical signals with different phase shifts caused by difference in the path length are recombined. When recombined, they interfere with each other to form outputs in different directions for different wavelengths. The spread signals are coupled into output ports 450. AWGs are manufactured using semiconductor photolithographic technologies. Although AWGs offer improved uniformity in insertion losses among channels, the insertion loss is generally high, often 5-7 dB. Moreover, AWGs typically suffer from high cross talk. In an AWG, the complexity and size of the device increases with increasing channel counts and decreasing channel spacing. The performance of AWGs is also temperature sensitive.
A filter for WDM applications should have a response curve as a function of wavelength that has a flat passband with steep skirts, what is descriptively known as a “brick-wall” or “boxcar” filter. A flat passband allows light within a tolerance of a desired wavelength to pass, and the steep skirts reduce the amount of out-of-band energy that passes, thus reducing cross talk.
FIG. 5 shows a sample spectrum 510 for a device with a narrow pass band 530 and a narrow stop band 540. For WDM optical networks, narrow pass and stop bands are problematic due to the physical limitations and temperature sensitivity of the signal transmitting laser devices. For example, the wavelength of light transmitted by a laser may not be exactly centered on a desired value. The amount that a wavelength is off center is referred to as offset. This offset is typically influenced by temperature. The amount of wavelength drift from the laser should not exceed the width of the pass band, otherwise a high insertion loss and a large amount of cross talk from neighboring channels occurs. Since it is difficult and expensive to produce lasers with high wavelength precision and stability, a wide passband is desirable. A brick-wall characteristic widens the passband while maintaining good out-of-band rejection.
Therefore, it is desirable to have a WDM with brick-wall filtering characteristics so as to allow high tolerance for wavelength offset and drift, and to reduce cross talk. The WDM should also be cost effective to manufacture, so it should be easy to align. The design should be such that losses do not accumulate, and insertion losses should be low.